Eons ago, mammals were mammoth, Yale researcher says

When the dinosaurs went extinct 65 million years ago, the land mammals that were left suddenly got huge -- and when we say suddenly, we're talking really fast, over a period of 20 million years.

In that relatively short time, by evolutionary standards, the mammals that remained after the dinosaurs died out grew much larger, according to a study released Thursday in the journal Science. Alison Boyer, a postdoctoral fellow at Yale University, is a co-author of the study.

Today's largest land animal, the African elephant, is smaller than those first gigantic mammals, likely because of competition from humans and other species, Boyer said.

"What's surprising is that it only took about 20 million years to reach the largest land mammal that's ever lived," Boyer said.

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That mammal, Indricotherium transouralicum, similar to a rhinoceros but lacking a horn, weighed 34,000 pounds and stood about 18 feet high at the shoulder. It lived on the Eurasian land mass.

Boyer, who works in the lab of Walter Jetz, part of Yale's department of ecology and evolutionary biology, was part of an international team of paleontologists, evolutionary biologists and macroecologists led by Felisa Smith of the University of New Mexico.

"What we did is, basically, went through almost all the fossil records of large mammals and made estimates of how big each species was," Boyer said. The team examined teeth and bones of odd-toed ungulates (horses and rhinos); elephants and their relatives, mammoths and mastodons; and the group that includes anteaters, tree sloths and armadillos.

While the evidence that dinosaurs' extinction enabled mammals to grow gigantic forms is circumstantial, Boyer said, the timing appears far more than coincidental.

Boyer said what's fascinating is that as each holder of the title of world's largest mammal faded into extinction, another would grow to take its place.

"One of the interesting patterns that we saw (was), after Indricotherium went extinct, some other unrelated group that wasn't another rhinoceros came up and took over the top spot," Boyer said. "About seven different groups have repeatedly evolved large size."

The key seems to be how much space the species had to roam in, and how cold the climate was -- really big species can hold in heat better than small ones and tend to inhabit cold climates.

The current title-holder is the elephant (although the blue whale beats it out for largest mammal on land or sea). But today's elephant is a shrimp compared with elephants of eons past.

"It's only relatively recently that they've taken over the top spot," Boyer said. "Up until 5 million to 2 million years ago, it was basically equal to Indricotherium, but today the elephant is a little bit smaller."

Even as far back as the Pleistocene epoch, which ended 12,000 years ago, mammals were larger than today: the earth shook with woolly mammoths, giant sloths and other supersize mammals ambling about.

Competition from other species, especially Homo sapiens, crowded out the larger animals. "There's only so much room ecologically for really big species," Boyer said.

But that doesn't mean other species couldn't grow larger, if circumstances were to change, such as humans' numbers suddenly dropping catastrophically.

"I think that, given enough time and given a decreased pressure from humans, big-bodied mammals would evolve again," Boyer surmised.